It is known that a capacitor element body with a very large apparent dielectric constant compared with a conventional ceramic dielectric can be obtained by insulating the crystal grain boundaries in a ceramic oxide semiconductor such as SrTiO.sub.3. It is also known that in a varistor where the current rises suddenly at threshold voltage can be obtained by forming electrodes on the element body. These elements are widely used for the circuits in electronic devices or the like to absorb noises. That is, low noise level of the element is advantageous in the application of it as a circuit element mounted with a capacitor while it can be used as a varistor to absorb large surge current.
For example, materials with large apparent dielectric constant of 2,000 to 100,000 are used as materials for a ceramic capacitor of grain boundary barrier type obtained by the following steps; diffusing cupric oxide (CuO) and bismuth trioxide (Bi.sub.2 O.sub.3) from the surface of the sintered body into a grain boundary of polycristalline ceramic semiconductor composed of SrTiO.sub.3 as a main component formed by adding Nb.sub.2 O.sub.5 and TiO.sub.2 -Al.sub.2 O.sub.3 -SiO.sub.2 compound and sintered in a reducing atmosphere, and forming a resistance layer in a grain boundary by forming a depletion layer in the crystal grain boundaries. The functions of CuO and Bi.sub.2 O.sub.3 as are often used as diffusion materials in a conventional manufacturing method are as follows: CuO containing sufficient oxygen forms a electron trap center in an crystal grain boundaries of the sintered body and traps electrons existing near the crystal grain boundaries of n-type semiconductor SrTiO.sub.3, thereby forming a depletion layer near the grain boundary where electrons do not exist. A capacitor is composed of the materials for a ceramic capacitor of grain boundary barrier type, charging electric loads on both sides of the insulated depletion layer thus formed.
On the other hand, it is known that varistor characteristics which raise the current suddenly at the threshold voltage or more appear when Na oxide or the like is added to a diffused material, Bi.sub.2 O.sub.3. This threshold voltage is also called the varistor voltage and is defined as the voltage that the current of 1 mA flows through varistor elements when a voltage is applied.
In the materials for the aforementioned ceramic capacitor of grain boundary barrier type, the apparent dielectric constant of the sintered body is approximately the value that the dielectric constant of SrTiO.sub.3 (approximately 200) is multiplied by the ratio of the grain diameter of SrTiO.sub.3 in the sintered body to the thickness of said grain boundary depletion layer (grain diameter / the thickness of grain boundary depletion layer). A typical thickness of the grain boundary depletion layer of the SrTiO.sub.3 sintered body is about 0.2 .mu.m. The standards of apparent dielectric constant are 2,000, 20,000, 200,000 corresponding to the grain diameters of the SrTiO.sub.3 sintered body of 2 .mu.m, 20 .mu.m, and 200 .mu.m, respectively.
Bi.sub.2 O.sub.3 which is added to form a barrier in a grain boundary is known as a good conductor of oxygen when Bi.sub.2 O.sub.3 is a .beta.-Bi.sub.2 O.sub.3 and .delta.-Bi.sub.2 O.sub.3 phase. When the surface of the sintered body is coated with Bi.sub.2 O.sub.3 and heat-treated, Bi.sub.2 O.sub.3 is first diffused along the grain boundary of the sintered body and then oxygen is transported by diffusion from outside to the inner portion of the sintered body along Bi.sub.2 O.sub.3 existing in the grain boundary. Bi.sub.2 O.sub.3 works to supply necessary oxygen to form the grain boundary depletion layer. This grain boundary barrier type of ceramic varistor having a high electrostatic capacitance displays excellent electrostatic capacitance and temperature characteristics and provides wide application fields in the industrial fields.
The aforementioned ceramic varistor having a high electrostatic capacitance of grain boundary barrier type is manufactured by the following steps; generally sintering is carried out at a high temperature so as to make the crystal grains in the sintered body as large as possible, by coating the sintered body with pasted cupric oxide and bismuth trioxide, etc. containing sodium oxide, followed by the heat treatment so as to diffuse such oxides as Bi.sub.2 O.sub.3, CuO, NaO into the inner portion of the sintered body and oxidize it.
When a ceramic capacitor with varistor characteristics having a high capacitance is manufactured by the method mentioned above it is required to uniformly diffuse Bi.sub.2 O.sub.3, Na.sub.2 O and CuO or the like from the surface of the sintered body into the inner portion thereof during the process. Nonuniform coating of diffused materials or the like is prone to cause variation in characteristics and, further, it is difficult to diffuse enough Bi.sub.2 O.sub.3, Na.sub.2 O and CuO or the like into the inner portion of the sintered body when it is thick, which causes problems such as limitation of the element size.
In addition, in a grain boundary barrier type ceramic varistor with large electrostatic capacitance, it is electrically required to have uniform characteristics and stability of characteristics against high voltage pulses, therefore a uniform material composition is required.
The present invention has an objective to provide a ceramic capacitor having varistor characteristics which solves these problems.